The present invention relates to a wind velocity measurement system that is specifically designed for measuring extreme wind velocities such as may occur, for example, during a hurricane.
Accurate high-speed wind information is needed at the National Aeronautics and Space Administration""s (NASA) Kennedy Space Center (KSC) to enable engineering assessment and re-certification of facilities, ground support equipment and flight hardware should an extreme wind event occur such as a hurricane, tornado or waterspout. For example, KSC has identified an operational requirement to record wind data during a hurricane under a scenario that involves a partial or complete Space Shuttle Solid Rocket Booster (SRB) stack exposed to a hurricane in the event that two complete vehicle stacks must be rolled back from the Pads in preparation for the storm. The requirement stems from a need to adequately understand and reconstruct the mechanical loads experienced by the SRB components in order to determine that the flight hardware is (or is not) safe to fly. The requirement is to capture wind pressures and directions, with a data rate of at least 50 samples per second, to facilitate reconstruction of SRB loads over the frequency range that encompasses the first and second bending modes. The National Oceanic and Atmospheric Administration (NOAA) also has a need for a wind direction and speed sensor that can be installed in the path of a hurricane to provide a reliable and accurate record of storm winds at ground level.
No commercial sensor system has been available to meet the NOAA and NASA requirements. In particular, recent studies of super storms have indicated that wind speeds in Category-5 hurricanes can exceed 200 mph while those in F-5 tornadoes can reach speeds in excess of 300 mph. The majority of conventional anemometers generally have relatively low upper speed limitations and are often hindered in performing at extremely high wind speeds because of their mass, aerodynamic drag, rotating mechanisms or sensitivity to other environmental factors such as water or acoustic noise.
More specifically, known devices for measuring wind velocity include rotating cup or propeller type anemometers, hot wire and hot film devices, acoustic anemometers and pitot type sensors. Rotating cup or propeller type anemometers have a spinning mass and high inherent aerodynamic drag that limits their high-speed performance. They also have a tendency to report winds higher than actual during the lulls in gusty conditions because the mass is slow to spin down. Hot wire and hot film devices, though low in drag, also tend to report higher than actual winds when operating in wet environments such as in hurricanes. These sensors typically do not discriminate between cooling of the sensing element due to wind or the additional cooling due to the water on the element. Acoustic anemometers, which measure wind-induced Doppler frequency shift of a locally transmitted tone, are often subject to erroneous output and signal masking due to high ambient noise levels due to turbulence at extremely high wind speeds. Single-orifice pitot type sensors, which have otherwise good high-speed performance, must be mechanically pointed into the wind. Omni-directional pitot devices are sometimes subject to water ingestion in the pressure tubes, which interferes with proper operation.
None of the foregoing wind velocity measurement devices are thus suitable for accurately measuring wind velocities over a wide range of values from relatively low speeds up to 300 mph from Category 5 hurricanes. Typically, any device that can accurately measure high-speed winds is not accurate at lower speeds, while those that are accurate at lower speeds cannot typically endure higher speed winds. A need therefore exists for a wind velocity measurement system that is rugged enough and accurate enough to be able to measure extremely high wind velocities, while at the same time possessing good low speed measurement sensitivity.
To address the foregoing need, the present invention provides a wind velocity measurement system that employs two different types of measurement techniques, both of which receive signals from a single measurement sensor. More particularly, the subject wind velocity measurement system employs the following different principles of physics to measure wind speed: (1) the aerodynamic force imparted to a low profile, rigidly mounted cylindrical rod, and (2) the vibrating frequency of the rod as vortices are shed from the rod""s cylindrical surface. In the preferred embodiment, a force sensor comprising a common set of strain gages is used for both measurements, and the strain gages generate signals in response to the force imparted by the wind on the rod. The signals generated by the strain gages are fed to processing circuitry that calculates the wind speed and direction from the signals. The force measurement is proportional to the square of the wind speed. Since it is a vector quantity it can also be used to derive wind direction. The vortex shedding frequency is a scalar quantity and is linearly proportional to wind speed. This frequency can be measured directly or calculated by analyzing the force measurements generated by the strain gages over time. Both of the wind velocity calculations can be advantageously used by the processing circuitry to generate an accurate wind velocity reading. In addition, the two measurements derived from the same sensor signals can be compared to one another to facilitate automatic checking of sensor calibration.